Internal combustion engines, including diesel engines, gasoline engines, gaseous fuel-powered engines, and other engines known in the art exhaust a complex mixture of air pollutants. These air pollutants are composed of gaseous compounds such as, for example, the oxides of nitrogen (NOX). Due to increased awareness of the environment, exhaust emission standards have become more stringent, and the amount of NOx emitted from an engine may be regulated depending on the type of engine, size of engine, and/or class of engine. In order to ensure compliance with the regulation of these compounds, some engine manufacturers have implemented a strategy called Selective Catalytic Reduction (SCR).
SCR is a process where gaseous or liquid reductant (most commonly a urea/water solution) is added to the exhaust gas stream of an engine and is absorbed onto a catalyst. The reductant reacts with NOx in the exhaust gas to form H2O and N2, which can be safely released to the atmosphere. Although SCR can be an effective method for reducing NOX, it can also be difficult to ensure that enough reductant has been injected to adequately reduce the amount of NOX present within the exhaust gas stream, without unnecessarily wasting reductant and releasing unused reductant or byproducts thereof to the atmosphere.
One attempt to regulate the injection of reductant is described in U.S. Pat. No. 6,361,754 (the '754 patent) issued to Peter-Hoblyn et al. on Mar. 26, 2002. The '754 patent discloses an exhaust system for reducing NOX emissions from an internal combustion engine. The exhaust system includes an exhaust pipe of an engine, and an SCR catalyst located within the exhaust pipe. The exhaust system also includes a tank of urea, a pump that pressurizes the urea, a valve that meters the pressurized urea, and a nozzle that injects the metered and pressurized urea into the exhaust pipe upstream of the SCR catalyst.
The injection of urea into the exhaust by the system of the '754 patent is regulated by a feedforward controller in response to a number of measured parameters, including: fuel flow, throttle setting, engine speed, rack setting, intake air temperature, barometric pressure, intake air humidity, and exhaust gas temperature. That is, the injection of urea based on target NOX concentrations can be preprogrammed into the feedforward controller based on tested values for given fuel flows and related parameters. In addition, to the extent the sensors are available, trim or feedback control is provided based on residual levels of gas species following the SCR catalyst, e.g., the level of NOX, HC, or CO. If desired, feedback control can also be employed to trim the system in response to residual levels of ammonia. In this manner, the sensors correct the preprogrammed values by feedback control.
Although perhaps somewhat effective at controlling reductant injections, the exhaust system of the '754 patent may fail to appropriately account for manufacturing variability and aging. That is, because the system relies on preprogrammed values, the accuracy of different systems preprogrammed with the same values may be low due to manufacturing variability. And, even if the system of the '754 patent provided preprogrammed values tailored to specific systems, the cost of doing the calibration testing on each system would be excessive. Further, even if extensive calibration is performed for each system, the feedforward control of the '754 patent may lose accuracy as the system ages. Without accounting for aging (both thermal and chemical aging), it may be difficult to diagnose system malfunction.
The system of the present disclosure solves one or more of the problems set forth above and/or other problems.